1. Field of the Invention
Exemplary embodiments of the invention relate to a gate shift register and a display device using the same.
2. Discussion of the Related Art
Various flat panel displays capable of reducing the weight and the size of a cathode ray tube have been developed and have been put on the market. In general, a scan driving circuit of the flat panel display sequentially supplies a scan pulse to scan lines using a gate shift register.
The gate shift register of the scan driving circuit includes a plurality of stages each including a plurality of thin film transistors (TFTs). The stages are cascade-connected to one another and sequentially generate an output.
Each of the stages includes a Q node for controlling a pull-up transistor and a Q bar (QB) node for controlling a pull-down transistor. Further, each of the stages includes a plurality of switching circuits for charging and discharging the Q node and the QB node to a predetermined voltage in response to a carry signal input from a previous stage, a carry signal input from a next stage, and a clock.
Such a related art gate shift register generates the scan pulse in only one direction, i.e., in only a direction from a stage positioned at an uppermost side to a stage positioned at a lowermost side. Thus, it is impossible to apply the related art gate shift register to various kinds of display devices, for example, a display device sequentially displaying an image in a direction from a lowermost scan line to an uppermost scan line of a display panel. The related art gate shift register does not satisfy various demands of companies of the display devices. Accordingly, a bidirectional gate shift register capable of performing a bidirectional shift operation has been recently proposed. The bidirectional gate shift register includes a bidirectional control circuit and operates in a forward shift mode or a reverse shift mode.
However, the bidirectional gate shift register causes several problems because of the bidirectional control circuit added to a unidirectional gate shift register. Because the bidirectional control circuit is floated after a shift direction conversion signal is applied to a discharge TFT connected between the QB node and an input terminal of a low potential voltage in each stage, a gate electrode of the discharge TFT is floated. Leakage charges are accumulated in the floated gate electrode of the discharge TFT during an operation of the gate shift register, and thus a voltage between the gate electrode and a source electrode of the discharge TFT exceeds a threshold voltage. As a result, the discharge TFT that has to be held in a turn-off state is abnormally turned on. In this case, the QB node is not charged to a voltage level capable of turning on the pull-down transistor during a period in which an output signal of the stage has to be held at a low level, and as a result, the output signal is not held at a gate low level and gradually increases. Further, a degradation of the discharge TFT accelerates because of a gate-bias stress resulting from the leakage charges, and a life span of the gate shift register shortens.